Carbon dioxide is one of the main gaseous emissions from the combustion process within an internal combustion engine. It is considered by many in the scientific community to be a major factor in global warming as a result of increase of greenhouse effect due to its emissions. Currently in Europe there are no mandatory emission standards for carbon dioxide emitted from passenger cars, but voluntary agreements with motor manufacturers are in place. In the UK financial measures have been introduced in 2001 in an attempt to lower carbon dioxide emissions by linking vehicle excise duty to carbon dioxide emission levels and type of fuel used. Therefore passenger car buyers pay lower annual vehicle excise duty for vehicles that emit lower levels of carbon dioxide.
However the European Union has now agreed a mandatory carbon dioxide target for passenger car manufacturers. Under this legislation car manufacturers have a fleet average emissions target for vehicles sold in Europe of 130 g CO2/km or below by 2015. This target is being gradually phased in from 2012. The US also recognises that the running of vehicles provides a major source of carbon dioxide emissions. Greenhouse Gas (GHG) emission standards have been set by the Environmental Protection Agency (EPA) and National Highway Traffic Safety Administration (NHTSA). These reductions are being phased in from 2009 to 2016 and are defined in terms of CO2— equivalents (g CO2/mile) whereby emissions of N2O and CH4 are included with multiplying factors of 296 and 23 respectively.
Therefore car manufacturers are striving to lower their carbon dioxide emissions by a variety of measures such as weight reduction, variable valve actuation, low friction components and stop-start technologies. By adopting such measures a reduction in CO2 emissions of 25-30% is reportedly achievable. Many of these measures will also lead to a reduction in the exhaust gas temperature because of the improved fuel efficiency.
The combustion process in the internal combustion engine of a vehicle is never perfect. Harmful emissions that result from the incomplete combustion are carbon monoxide, unburned hydrocarbons and NOx. There are existing and future emission standards for such gaseous emissions. Carbon monoxide and hydrocarbons are typically removed from the exhaust gas of the internal combustion engine by use of an oxidation catalyst as part of a catalytic convertor. In simple terms a catalytic convertor needs to provide a structure that exposes a maximum surface area of catalyst to the exhaust gas stream and the catalyst needs to aid the reaction of the carbon monoxide and hydrocarbons with oxygen in the exhaust gas stream. Also the cost of the catalyst must be minimised, for example by using less and/or using less expensive materials.
Catalysts that have been used most successfully for oxidation reactions in catalytic convertors are precious metals, specifically platinum, which is a very expensive material. Palladium has been combined with platinum to reduce the catalyst costs and also has been found to reduce sintering of the platinum at higher temperatures. However palladium itself is known to have lower reactivity under very oxidising (lean oxidising) conditions relative to platinum. Unlike platinum, which has a higher ionisation potential and lower oxide stability, palladium exists mostly as an oxide with low specific activity for the oxidation of carbon monoxide and hydrocarbons.
Palladium is also known for its ability to react with sulphur dioxide, present in diesel exhaust gases, to form a stable sulphate which requires high temperatures to decompose. The decomposition of palladium sulphate in a lean environment requires temperatures in excess of 700° C. or lower temperatures in rich fuel gas exhaust but then there is a fuel penalty because of the creation of the rich environment. Although there have been moves across the world to reduce the level of sulphur present in diesel fuel (currently the mandatory level in Europe is 10 ppm) sulphur poisoning of exhaust gas catalysts is still an issue.
WO2010/090841 A1 discloses preparation of a palladium gold catalyst which is supported on alumina. The palladium and gold particles are described as being “in close contact”. There is no mention of whether they are or can be present as an alloy. Pd—Au (wt % Au—Pd of ˜1:1.5) on alumina were exposed to a gas mixture having the composition: 1000 ppm CO, 225 ppm propene, 105 ppm propene, 450 ppm NO, 10% O2 and He balance and heated to 673 K at 10K/min. FIGS. 7A-B and 8A-8B show the oxidation profiles of carbon monoxide and propene respectively with light off temperatures which all seem to be above 150° C. It appears that the choice of metal oxide support for the Pd and Au particles has not been deemed important to the optimisation of the catalyst activity nor is the formation of the alloy between the particles.
WO2009/136206 A1 discloses an exhaust system for lean-burn internal combustion comprising a Pd—Au alloy catalyst on a metal oxide support. There is no disclosure in the patent specification of possible types of metal support but alumina is used in all the Examples. A variety of catalysts were exposed to a gas mixture having the composition 1000 ppm CO, 900 ppm hydrocarbon, 200 ppm NO, 2 ppm SO2, 12% O2, 4.5% CO2, 4.5% H2O and N2 balance. Data for a variety of Au:Pd ratios show temperatures for 80% carbon monoxide conversion and temperatures for 50% hydrocarbon conversion of at least 150° C.
EP 0602865 A1 discloses noble metal-metal oxide catalysts prepared by co-precipitation and their use to catalyse the oxidation of carbon monoxide and hydrocarbons in internal combustion engine exhaust gas. The metal oxide comprises one or more of ceria, zirconia, titania or stannic oxide with ceria being especially preferred. Noble metals disclosed include one or more of ruthenium, rhodium, palladium, osmium, iridium, platinum and gold. It is disclosed in the specification that catalysts tested on simulated car gas exhaust which is rich of stoichiometric at a lambda value of 0.98 convert one or more of a) 50% CO at a temperature lower than 250° C., preferably lower than 150° C. b) 50% of nitrogen monoxide at a temperature lower than 300° C., preferably lower than 250° C. and c) 50% of hydrocarbons present as propene at a temperature lower than 350° C., preferably lower than 300° C. The Examples disclosed include Pd-ceria, Pt-ceria, Pd—Pt-ceria, Pd—Pt-ceria-alumina, Pt-stannic oxide and Au-zirconia.
WO2004/025096 A1 discloses a supported palladium catalyst for homogeneous charge compression ignition diesel engine (HCCI). This engine differs from a conventional direct injection diesel engine in that all fuel for combustion is injected into the combustion chamber prior to the start of combustion. Such engines were found to produce high levels of CO and relatively high levels of HC on combustion compared to conventional direct injection diesel engines. The examples show temperature of conversion for a variety of catalysts according to the invention for exhaust gas emissions from both HCCI and direct injection diesel engines. For direct injection engines Pt-alumina was superior to Pd-ceria for low temperature conversion of both CO and HC (Example 1).